1. Field of the Invention
The present invention relates to a laser diode driving device for an optical disc.
2. Description of the Related Art
FIG. 4 shows an example of a conventional laser diode driving device (e.g., see JP 63 (1988)-143887 A). In FIG. 4, an input PchMOS transistor 105 constituting a mirror circuit of a PchMOS transistor is connected to a pull-in type constant current source 103 through a switch 104, and an output PchMOS transistor group 106 constituting the mirror circuit of the PchMOS transistor 105 is connected to a laser diode 107.
When the switch 104 is closed, an output current that is a gain-fold of the mirror circuit flows to the laser diode 107. On the other hand, when the switch 104 is opened, the output current flowing to the laser diode 107 stops flowing.
However, with the above-mentioned conventional configuration, the following problems arise.
In general, the drive current of a laser diode is large. Therefore, the size of the output PchMOS transistor group 106 constituting the mirror circuit of the PchMOS transistor is large. Because of this, the gate capacitance of the PchMOS transistor is large, which makes it impossible to decrease a gate potential abruptly when the switch 104 is closed. Consequently, the rising time of the drive current of the laser diode 107 is long. Furthermore, when an input current is decreased so as to adjust a light-emission power of the laser diode 107, the rising time of the drive current of the laser diode 107 is long. In contrast, when the switch 104 is opened, the gate potential does not rise abruptly, so that the falling time of the drive current of the laser diode 107 is long.
Recently, the storage capacity of an optical disc is increasing. In order to further increase the storage capacity in such a situation, it is necessary to shorten the rising time and falling time of a drive current of a laser diode. However, the difficulty in accomplishing this makes it difficult to increase the storage capacity of an optical disc in the above-mentioned conventional laser diode drive device.